1. Field of the Invention
The present invention relates generally to a method of making a polypropylene printing plate, and is directed to a method of making a polypropylene printing plate using a paper matrix.
The present invention is also directed to a reproducing printing plate made of a crystalline ethylene-propylene copolymer.
2. Description of the Prior Art
In order to print a newspaper, it is essentially required to produce a large number of newspapers at high speed. In the prior art, a so-called paper matrix-stereotype method has been employed in which a paper matrix is formed based upon an original plate, and molten lead alloy is cast into the paper matrix to form a reproduction printing plate which is mounted to a rotary press to carry out the printing of a newspaper. In the labor environment to handle lead alloy, there are many problems, which should be improved, such as those of operation at high temperature, lead poisoning, weight transportation and the like. Thus, there has recently been proposed a method of forming a printing plate by using thermoplastic resin. Its practical method is to make a printing plate of thermoplastic resin by using a paper matrix similarly as used in the prior art paper matrix-stereotype method in accordance to, for example, Japanese patent application publication No. 12933/1973, Japanese Pat. Laid-Open No. 33704/1974 or Patent Laid-Open No. 3959/1974. In this case, it has been known that as the thermoplastic resin it is most preferred to employ crystalline polypropylene whose melt flow index (hereinafter referred to as MFI) (JIS-K6758) is 3.about.10 and as the paper matrix it is suitable to employ one such as disclosed in Japanese utility model application publication No. 27601/1971 whose surface layer is superior in stripability and strongly sticks to its base body.
However, in the above described method, that is, the method of making a reproduction printing plate comprising the steps of mounting a paper matrix to either of a roll and its opposing member, supplying melt of crystalline polypropylene of MFI 3.about.10 to a clearance between the roll and opposing member, pressing the melt to the matrix, and cooling the melt to be hardened, there is a great problem in that the paper matrix life should be improved. That is, at a newspaper company having a large circulation, since a necessary number of newspapers must be printed and sent out for circulation in a limited short time period, it is required to operate a number of rotary presses for printing at the same time and hence many reproduction printing plates are needed. For example, according to a questionaire regarding the inspection standard for receiving paper flongs at newspaper plants (the interim report of Research Society, Special Technical Meeting, 1963, Japan Press Association), preferable number of casting the stereotype per one paper matrix in the paper matrix-stereotype printing method was more than 20 for 80% of plants, more than 30 for 60% of plants and more than 40 for 18% of plants.
When a polypropylene printing plate is made, such a paper matrix as disclosed in the above mentioned Japanese utility model application publication No. 27601/1971 is preferably employed, in which the surface layer of the paper matrix is selected to be superior in stripability and also the contact between the surface layer and base body of the paper matrix is made strong. However, there has not yet been sold such a paper matrix which is strong enough to always endure reproduction of 20.about.30 or more resin printing plates without being affected by conditions occurring in the case of pressing paper flongs on an original plate such as etched metal plate, type form, photopolymer plate or the like so as to make paper matrices. At present, when printing plates are reproduced by crystalline polypropylene, the paper matrix is endurable generally for about 15 plates and normally for about 7.about.10 plates.
In order to prepare a reproduction printing plate which faithfully reproduces even the details of a matrix, a resin having excellent flow property must be employed. Polypropylene of MFI 3.about.10, preferably of MFI 5.about.8 is suitable for the above purpose. However, a paper matrix mainly uses fiber in its material so that a great number of fine bores are formed therethrough. For this reason, when molten resin is pressed into concave portions of the paper matrix, the molten material is intruded into the fine bores to make the resin printing plate difficult to be stripped therefrom so that the paper matrix is damaged to decrease the number of reproduction printing plates. Thus, in order to further facilitate stripability of the paper matrix, it is sometimes coated with a suitable surface lubricant after the production of paper matrix. In this case, however, there is a drawback such that the fine bores of the paper matrix are clogged up by the surface lubricant to deteriorate the reproduction fidelity of printing plates. These fine bores function to exhaust therethrough air confined between the matrix and molten resin when a printing plate is formed, so that they are indispensable for providing high fidelity to the printing plate. Accordingly, maintaining air permeability of a paper matrix to enhance the reproduction fidelity of printing plates and keeping excellent stripability to improve the matrix life are necessary conditions for developing the paper matrix-resin printing method widely in the newspaper printing.